Transportation systems are an integral part of an industrial society. By providing mobility, transportation systems enable the trading of goods and convenience for users. These systems require energy to operate. The increase in population has resulted in more energy consumption by transportation systems. The cause for a cleaner environment and conservation of energy sources has led to the integration of more electric vehicles in the transportation systems. This has been accomplished by introducing hybrid electric vehicles and electric vehicles.
Currently, the majority of the hybrid electric vehicles utilize permanent magnet motor drives for propulsion because these drives have high torque densities resulting from the field created by the permanent magnets. However, the increasing demand for rare earth materials used in the manufacture of high energy magnets leads to a shortage of these elements and increased costs. Therefore, there is a demand for new types of motors with limited or no use of permanent magnets for automotive applications.
Switched reluctance machines have high torque densities, which can provide a solution for replacing permanent magnet machines. However, drives of the prior art for switched reluctance machines having n number of phases have a high number of semiconductor switches which will increase the cost of the converter dramatically. Further, the drives of the prior art require a high number of cable connections to the switched reluctance machines, which is a drawback for automotive applications of switched reluctance machines.
Referring to FIG. 1A for example, asymmetric bridge 101 of the prior art is shown. This topology requires 2×n switches and 2×n diodes for an n-phase switched reluctance machine. In order to increase efficiency, soft-switching inverters have been utilized in high speed applications where the switching frequency of the inverter is high. In conventional applications, for a switched reluctance machine having n phases, 2×n lead wires are required. In automotive industries, especially in powerful traction applications, the number of wires is an important factor because of the increased cost of manufacturing and maintenance per added wire. Therefore, the use of asymmetric bridges in automotive applications increases the costs of manufacturing.
Referring to FIG. 1B in another example, C-dump inverter 102 of the prior art is shown. Star connection 103 connects to buck converter 104 to provide a secondary voltage source for the operation of the drive. Dump capacitor 105 is the input capacitor for buck converter 104. Dump capacitor 105 is used for dumping the magnetizing energy of each phase at the end of the corresponding conduction cycle. C-dump inverter 102 requires n+1 switches and n+1 diodes for a switched reluctance machine having n phases. However, C-dump inverter 102 requires dump capacitor 105 or a variable voltage structure to maintain operational conditions. Further, dump capacitor 105 is bulky and requires cooling for operating at high temperatures. Therefore, the use of C-dump inverter 102 in automotive applications is costly and an inefficient use of space.
Therefore, there is a need in the prior art for a low cost drive and control method for switched reluctance machines with n number of phases that reduces the number of cable connections to the switched reluctance machines and prioritizes different phases in applying a reference current in the phases, thereby allowing for safe multi-phase operation of the drive of the switched reluctance machine.